linux-kernel - Re: [PATCH] ARM: SWP emulation: Restore original *data when failed

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Message-ID: <561F6F3B.6030607@arm.com>
Date:	Thu, 15 Oct 2015 10:17:47 +0100
From:	Vladimir Murzin <vladimir.murzin@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Shengjiu Wang <shengjiu.wang@...escale.com>
CC:	"ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"nico@...aro.org" <nico@...aro.org>,
	Punit Agrawal <punit.agrawal@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH] ARM: SWP emulation: Restore original *data when failed

On 15/10/15 09:57, Russell King - ARM Linux wrote:
> On Thu, Oct 15, 2015 at 04:36:31PM +0800, Shengjiu Wang wrote:
>> On Thu, Oct 15, 2015 at 09:24:17AM +0100, Russell King - ARM Linux wrote:
>>> On Wed, Oct 14, 2015 at 10:51:17AM +0800, Shengjiu Wang wrote:
>>>> __user_swpX_asm maybe failed in first STREX operation, emulate_swpX
>>>> will try again, but the *data has been changed in first time. which
>>>> cause the result is wrong. So need to recover the *data when failed.
>>>>
>>>> Signed-off-by: Shengjiu Wang <shengjiu.wang@...escale.com>
>>>> ---
>>>>  arch/arm/kernel/swp_emulate.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/arch/arm/kernel/swp_emulate.c b/arch/arm/kernel/swp_emulate.c
>>>> index 5b26e7e..c61fbf92 100644
>>>> --- a/arch/arm/kernel/swp_emulate.c
>>>> +++ b/arch/arm/kernel/swp_emulate.c
>>>> @@ -41,6 +41,7 @@
>>>>  	"1:	strex"B"	%0, %2, [%3]\n"			\
>>>>  	"	cmp		%0, #0\n"			\
>>>>  	"	movne		%0, %4\n"			\
>>>> +	"	movne		%1, %2\n"			\
>>>>  	"2:\n"							\
>>>>  	"	.section	 .text.fixup,\"ax\"\n"		\
>>>>  	"	.align		2\n"				\
>>>
>>> I think I'd prefer this to be:
>>>
>>> 	__asm__ __volatile__(					\
>>> 	"0:	ldrex"B"	%2, [%3]\n"			\
>>> 	"1:	strex"B"	%0, %1, [%3]\n" 		\
>>> 	"	cmp		%0, #0\n"			\
>>> 	"	moveq		%1, %2\n"			\
>>> 	"	movne		%0, %4\n"			\
>>>
>>> so that we're not loading into %1 (an in-out non-temporary) but rather
>>> loading it into a temporary - and only overwriting the saved register
>>> value if the swap succeeds.
>>>
>>> Thanks.
>>
>> I am ok with your change. Need I send another patch for this change? or will
>> you send it by yourself?
> 
> Please send a new patch, thanks.
> 

We might need the same change for arm64 counterpart (see
arch/arm64/kernel/armv8_deprecated.c).

Vladimir

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